Q 

143 

L5 

S88 

PHYS 


UC-NRLF 


B   M   n?   EH'^ 


SKETCH    OF 
PROF.    JOHN    LE    CONTE 


SENSITIVE    FLAMES    AND 
SOUND-SHADOAYS 


BY 

WALTER  LE  CONTE  STEVENS 

PROFESSOR    OF    PHYSICS    IN     THF.    PACKEK    COLLEGIATE    INSTITDTE 


TWO   ARTICLES 

REPRINTED  FROM  THE  POPULAR   SCIENCE  MONTHLY 

FOR  NOVEMBER,  18S9 


NEW    YOKK 
IJ.    APPLE  TON    AND    COMPANY 

1889 


TS 


JOHN    LE    CoXTE 


SKETCH    OF 
PROF.    JOHN    LE    CONTE 


SENSITIVE    FLAMES    AND 
SOUND-SHADOWS 


BY 

WALTER  LE  CONTE  STEVENS 

PEOPESSOR    OF    PHYSICS    IN    THE    PACKER    COLLEGIATE    INSTITDTK 


TWO  ARTICLES 

HEPRmTEB  FROM  THE  POPULAR  SCIENCE  MONTHLY 

FOR  NOVEMBER,  1889 


NEW   YOKE 
D.    APPLETON    AND    COMPANY 

1889 


v;h  ^ccrxss/o^^^ 


SKETCH  OF  PROF.  JOHN  LE  CONTE. 

By  Prof.  W.  LE  CONTE  STEVENS. 

THE  subject  of  the  present  sketch  is  the  Professor  of  Physics 
in  the  University  of  California,  where  he  has  for  many  years 
been  associated  with  his  brother,  the  distinguished  geologist  and 
writer  on  evolution.  He  was  the  second  son  of  Louis  Le  Conte, 
and  was  born  on  the  4th  of  December,  1818,  at  the  family  home- 
stead in  Liberty  County,  Georgia.  The  father  was  a  man  of 
much  independence  of  character,  firm  and  decided,  yet  kind  and 
gentle,  exceedingly  fond  of  investigation,  original  in  thought,  but 
singularly  indifferent  to  popular  recognition.  He  published  noth- 
ing himself,  and  would  never  have  become  known  away  from  his 
own  home,  had  not  others  been  appreciative  enough  of  his  real 
merit  to  give  some  of  his  results  to  the  world  by  presenting  them 
before  the  New  York  Lyceum  of  Natural  History. 

By  personal  influence  and  example,  Louis  Le  Conte  inculcated 
in  his  sons  the  love  of  science,  and  of  truth  for  its  own  sake.  The 
virtue  of  verification  was  one  which  he  sought  to  cultivate  in 
them  as  of  cardinal  importance.  An  illustration  of  the  success 
of  his  teaching  in  this  direction,  and  of  the  early  growth  of  the 
philosophical  habit  of  mind  in  his  son  John,  was  afforded  on  one 
occasion  when  the  father  and  a  number  of  neighbors,  while  pa- 
trolling at  night  to  check  some  illicit  transactions  between  the 
negro  slaves  and  the  shopkeepers  of  the  nearest  village,  were  fired 
upon  with  blank  cartridges,  and  thrown  from  their  startled  horses. 
Relating  the  story  of  his  mishap  after  he  had  reached  home,  the 
father  said,  "  I  lost  my  left  stirrup ;  at  the  turn  in  the  road  I  lost 
the  other  stirrup,  and  at  the  next  turn  I  was  thrown."  John,  who 
listened  to  the  narrative  with  great  interest,  was  perplexed  to 
know  how  the  stirrups  could  have  been  lost.  His  night's  rest  did 
not  remove  the  trouble,  and,  leaving  his  bed  before  sunrise,  he 
went  and  examined  the  saddle.  He  reported  upon  the  result  of 
his  investigation  at  the  breakfast-table.  "  Pa,  did  you  not  say 
last  night  that,  when  the  horse  ran  away  with  you,  you  lost  your 
stirrups  ?  "  "  Yes,  my  son,  I  did  say  so."  "  Well,  I  have  found 
that  the  stirrups  are  safe  and  sound."  The  laugh  was  turned 
against  the  son,  and  the  father  often  told  the  story  afterward  as  a 
joke  upon  him.  It  was,  however,  no  joke  ;  it  was  a  prediction  of 
the  career  of  the  future  investigator  in  physics. 

The  childhood  and  most  of  the  boyhood  of  John  Le  Conte  were 
spent  at  the  plantation  home  in  Georgia,  where  hunting,  fishing, 
boating,  and  all  kinds  of  athletic  sports  contributed  largely  to  the 
training  of  his  observing  faculties.     His  uncle.  Major  Le  Conte, 


4        SKETCH   OF  PROF.  JOHN  LE   CONTE. 

an  accomplished  zoologist,  often  gave  up  liis  New  York  home  in 
winter  for  the  jnirpose  of  spending  the  colder  months  on  the 
Southern  plantation.  The  scientific  proclivities  of  both  father 
and  uncle  insensibly  made  all  the  children  students  of  natural 
history  and  collectors  of  specimens.  Thus  they  gradually  imbibed 
knowledge  on  such  subjects,  and  acquired  powers  of  discrimina- 
tion that  are  ordinarily  attained  only  by  years  of  study  in  maturer 
life.  Their  mother  died  in  1826,  leaving  the  father  in  charge  of 
six  children.  Deprived  of  maternal  care  at  so  early  a  period  of 
life,  all  of  them,  and  especially  the  boys,  were  thrown  largely 
upon  their  own  resources  at  a  tender  age. 

In  those  days  and  in  that  country  neighborhood,  forty  miles 
from  the  nearest  city,  Savannah,  it  was  necessary  to  do  without 
the  school  accommodations  that  are  now  abundant  in  every  vil- 
lage of  our  land.  An  isolated  wooden-framed  house,  with  no 
plastering,  a  single  door  for  its  single  room,  abundant  ventilation 
through  the  crevices  of  the  floor  and  walls,  fully  supplemented 
by  the  draught  through  an  ample  clay  chimney — such  was  the 
scliool-house  in  which  the  children  were  gathered  daily  from 
plantations  varying  in  distance  from  one  to  half  a  dozen  miles  or 
more.  The  teacher  was  rarely  ever  of  the  best.  One  there  was 
who  took  charge  of  this  road-side  seminary  for  two  years,  became 
the  intimate  friend  of  Mr.  Le  Conte,  and  exerted  over  his  boys  an 
influence  that  became  life-long.  Alexander  H.  Stephens,  the  fu- 
ture statesman  and  historian)  was  then  a  young  graduate  who 
sought  in  teaching  the  pecuniary  support  that  was  necessary 
while  he  was  preparing  for  admission  to  the  bar.  His  fine  classi- 
cal taste  and  clear,  logical  mind  jproduced  a  lasting  impression 
upon  John  Le  Conte,  who  received  thus  his  training  for  college^ 
and  entered  Franklin  College,  now  the  University  at  Athens,  Ga., 
with  distinguished  success  in  January,  1835. 

As  a  student,  young  Le  Conte  soon  became  noted  for  his  clear- 
ness of  conception  and  his  scrupulous  accuracy  in  work.  The 
curriculum  of  study  was  the  same  for  all,  irresi^ective  of  native 
bias  or  prospective  aim  in  life.  He  was  fully  appreciative  of  all 
the  classical  culture  that  was  there  afforded,  but  his  tastes  natu- 
rally led  him  into  spending  on  mathematics  and  its  applications 
a  larger  share  of  attention  than  Latin  and  Greek  could  attract. 
"  Give  him  the  cosine  of  A  and  he  will  prove  anything,"  was  the 
criticism  expressed  by  an  admiring  fellow-student,  and  concurred 
in  by  the  rest.  The  formal  teaching  of  physics  and  chemistry 
involved  mere  text-book  recitation,  and  attendance  upon  illus- 
trated lectures  of  the  most  elementary  character,  which  were 
delivered  with  oracular  authority.  It  was  more  than  whispered 
among  the  students  that  on  these  topics  John  Le  Conte  knew  a£ 
much  as  or  more  than  tlie  professor  himself. 

vol.   XXXVI. — 8 


SKETCH   OF  PROF.   JOHN  LE   CONTE. 


5 


During  his  senior  year  at  college  Mr.  Le  Conte  was  bereft  of 
his  devoted  father,  who  died  after  a  very  brief  illness.  This  ca- 
lamity hastened  his  selection  of  a  profession.  In  August,  1838, 
he  was  graduated  with  high  honor.  Immediately  afterward  he 
began  the  study  of  medicine,  and  in  the  spring  of  1839  he  entered 
the  College  of  Physicians  and  Surgeons  in  New  York,  where,  in 
March,  1841,  he  received  the  degree  of  Doctor  of  Medicine.  A  few 
months  before  his  graduation  in  medicine  another  domestic  calam- 
ity befell  him  in  the  death  of  his  eldest  brother,  William,  to  whom 
had  been  committed  the  charge  of  the  family  estates  in  Georgia. 
This  event  hastened  Dr.  Le  Conte's  return  home  in  the  spring  of 
1841,  to  take  charge  of  the  estate  as  the  eldest  surviving  son,  and 
frustrated  the  execution  of  a  cherished  plan  for  supplementing 
his  medical  education  by  a  year's  residence  in  Paris. 

During  the  summer  of  1841  Dr.  Le  Conte  returned  to  New  York, 
and  was  married  in  July  to  Miss  Josephine  Graham,  of  that  city, 
an  accomplished  young  lady  of  Scottish  and  English  extraction. 
The  deep  love  and  earnest  devotion,  and  the  consequent  domestic 
happiness  which  crowned  this  union,  contributed  more  than  all 
else  afterward  to  fortify  and  sustain  him  in  the  battle  of  life.  Mrs. 
Le  Conte  was  a  woman  of  wonderful  personal  magnetism,  queenly 
in  bearing,  and  of  extraordinary  beauty.  Her  brilliancy  and  wit, 
her  quick  insight  and  ready  tact,  added  to  her  majestic  presence, 
made  her  the  center  of  attraction  in  every  social  gathering.  In 
after-years,  especially  at  the  annual  meetings  of  the  American  As- 
sociation for  the  Advancement  of  Science,  such  men  as  Bache, 
Peirce,  Henry,  and  Agassiz  vied  with  each  other  in  doing  her 
homage.  Her  fame  in  social  circles  equaled  that  of  her  husband 
among  men  of  science ;  and  no  important  step  in  his  life  has  been 
taken  without  acknowledgment  of  the  help  derived  from  the  so- 
cial influence  of  a  wife  of  whom  he  was  justly  proud. 

In  the  autumn  of  1842  Dr.  Le  Conte  established  himself  as  a 
practitioner  of  medicine  in  Savannah,  Georgia.  His  four  years  of 
residence  in  that  city  formed  no  exception  to  the  usual  experience 
of  a  young  doctor  :  a  very  small  practice  and  an  increasing  fam- 
ily. It  afforded,  however,  an  excellent  opportunity  for  study  and 
research,  and  it  was  during  this  period  that  he  made  his  most  im- 
portant contributions  to  medical  literature.  These  at  once  estab- 
lished his  reputation  in  the  profession  as  an  acute  observer,  cau- 
tious, exact,  and  industrious.  The  first  of  them,  entitled  "  A  Case 
of  Carcinoma  of  the  Stomach,"  published  in  the  "  New  York  Med- 
ical Gazette  "  in  1842,  was  the  initial  outcome  of  a  series  of  obser- 
vations on  cancer  that  has  been  continued  from  time  to  time,  even 
after  Dr.  Le  Conte's  abandonment  of  the  practice  of  medicine.  At 
this  period  he  probably  paid  more  attention  to  physiology  than  to 
an^^  other  of  the  departments  included  in  medical  science,  and  his 


6        SKETCH   OF  PROF.  JOHN  LE   CONTE. 

fondness  for  research  interfered  to  some  extent  with  the  efforts 
that  might  have  been  made  to  secure  paying  patients. 

In  August,  184G,  Dr.  Le  Conte  accej^ted  the  chair  of  Natural  Phi- 
losophy and  Chemistry  in  Franklin  College,  his  alma  mater,  from 
which  he  had  gone  forth  eight  years  before  as  the  best  scientific 
student  in  his  class.  This  decided  his  withdrawal  from  the  field 
of  practical  work  in  medicine.  Henceforth  he  devoted  himself  to 
the  study  of  physical  science,  but  without  failing  to  keep  pace 
still  with  the  progress  of  physiology.  He  retained  his  professor- 
ship at  Athens  for  nine  years,  resigning  it  in  the  autumn  of  1855 
to  become  lecturer  on  chemistry  in  the  College  of  Physicians  and 
Surgeons,  his  medical  alma  mater.  In  the  spring  of  1856,  at  the 
conclusion  of  his  course  of  lectures  in  New  York,  he  accepted  a 
call  to  the  South  Carolina  College  at  Columbia,  where  he  had  been 
unanimously  elected  to  fill  the  chair,  then  first  created,  of  Natural 
and  Mechanical  Philosophy.  This  position  he  held  until  the  col- 
lege was  disbanded  soon  after  the  opening  of  the  civil  war.  He 
was  then  put  in  charge  of  the  Niter  and  Mining  Bureau  of  South 
Carolina.  In  1866  the  University  of  South  Carolina  was  organ- 
ized, and  Dr.  Le  Conte  was  elected  to  the  same  chair  that  he  had 
held  in  the  college  of  which  this  was  the  new  development.  This 
position  he  retained  until  1869,  when  he  gave  up  his  residence  in 
Columbia  to  become  an  adopted  citizen  of  California.  Here  his 
home  has  continued  up  to  the  present  time. 

The  period  of  thirteen  years  embracing  Dr.  Le  Conte's  connec- 
tion with  the  South  Carolina  College  and  University,  although 
clouded  by  the  saddening  events  incident  to  the  civil  war,  con- 
stituted the  pleasantest  and  most  satisfactory  period  of  his  life. 
The  institution  was  governed  by  a  board  of  trustees  composed  of 
gentlemen  of  refinement  and  culture,  who  entertained  a  genuine 
sympathy  for  the  labors  of  the  student  who  strives  to  plant  him- 
self at  the  most  advanced  outposts  of  science  and  literature.  The 
community  amid  which  the  college  had  been  developed  was 
strongly  influenced  by  the  atmosphere  of  scholarship  which  it 
produced.  There  was  a  quiet  spirit  of  encouragement  to  learn- 
ing, which,  by  its  freedom  from  pretension,  furnished  the  most 
grateful  incentive  to  study.  It  was  during  these  years  that  Dr. 
Le  Conte  established  a  European  reputation  through  his  writings, 
which  were  publislied  cliiefly  in  the  "  American  Journal  of  Sci- 
ence "  and  the  "  London  Philosophical  Magazine."  It  was  in  1857 
that  he  made  the  remarkable  discovery  of  the  sensitiveness  of 
flame  to  musical  vibrations — a  discovery  which  served  as  the 
starting-point  for  Barrett,  Tyndall,  and  Koenig  in  the  exquisite 
applications  that  have  since  been  worked  out  by  the  use  of  flame 
for  the  detection  of  sounds  too  delicate  for  the  ear  to  perceive, 
and  for  the  optical  analysis  of  comj^ound  tones.     Unfortunately, 


' 


SKETCH   OF  PROF.   JOHN  LE   CONTE.  7 

Dr.  Le  Conte  did  not  possess  the  wealth  of  instrumental  appli- 
ances needed  for  the  development  of  his  unique  discovery,  but  his 
priority  was  gracefully  proclaimed  by  Tyndall  in  the  now  classic 
book  on  sound,  made  up  of  lectures  delivered  at  the  Royal  Insti- 
tution. Among  other  papers  that  attracted  marked  attention  in 
Europe  was  one  "  On  the  Adequacy  of  Laplace's  Explanation  to 
account  for  the  Discrepancy  between  the  Computed  and  the  Ob- 
served Velocity  of  Sound  in  Air  and  Gases,"  written  in  1861  and 
published  in  1864.  Laplace's  modification  of  Newton's  formula 
had  been  questioned  by  eminent  English  mathematicians  and 
physicists.  Dr.  Le  Conte  showed  that  the  obscurity  into  which 
the  subject  had  been  thrown  was  due  to  misconception  of  the 
physical  theory  of  Laplace,  and  to  the  difficulties  and  obscurities 
which  invest  the  mathematical  theory  of  partial  differential  equa- 
tions in  their  application  to  physical  questions.  This  paper  evoked 
replies  from  Profs.  Challis,  Earnshaw,  and  Potter,  in  England ; 
but  the  American  physicist's  position  is  generally  accepted  to- 
day. The  paper  is  a  model  of  exact  physical  reasoning.  In  addi- 
tion to  the  discussion  of  Laplace's  views,  it  contains  an  original 
investigation  of  the  bearing  of  the  phenomena  attending  the 
propagation  of  sound  in  air  on  the  question  whether  the  gases 
constituting  our  atmosphere  are  in  a  state  of  mixture  or  of  com- 
bination. 

Just  before  the  close  of  the  war  the  home  of  Dr.  Le  Conte  was 
included  in  the  belt  of  desolation  that  was  left  by  General  Sher- 
man's march  through  South  Carolina.  Among  the  losses  by  fire 
was  the  manuscript  of  a  volume  on  general  physics,  the  product 
of  Dr.  Le  Conte's  many  years  of  experience  as  a  teacher  and  stu- 
dent of  this  subject.  The  tribulations  of  the  reconstruction  period 
in  South  Carolina  during  the  years  following  the  war  made  sci- 
entific investigation  impossible.  The  political  turmoil,  and  the 
inauguration  of  the  rule  of  ignorance  and  vice  in  place  of  intel- 
ligence, left  no  refuge  but  expatriation  for  those  whose  occupa- 
tions depended  upon  the  embellishments  of  civilization.  To  this 
source  of  disquietude  was  added  the  burden  of  domestic  affliction 
in  the  loss  of  an  only  daughter  in  the  bloom  of  early  womanhood. 

At  this  critical  time  came  a  call  to  the  Pacific  coast,  to  assume 
the  chair  of  Physics  and  Industrial  Mechanics  in  the  University 
of  California,  which  was  then  in  the  incipiency  of  its  organiza- 
tion. The  offer  was  accepted,  and  Dr.  Le  Conte  arrived  in  San 
Francisco  in  April,  1869.  Being  immediately  appointed  acting 
president,  he  drew  up  the  first  prospectus  of  the  university,  in 
which  was  set  forth  a  synopsis  of  the  proposed  courses  of  instruc- 
tion. In  September  of  the  same  year  exercises  were  begun  in 
temporary  buildings  at  Oakland,  where  during  the  following  sum- 
mer he  conferred  the  baccalaureate  degree  on  three  young  men, 


8   *     SKETCH   OF  PROF.  JOHN  LE   CONTE. 

and  then  retired  from  executive  duties  in  order  to  build  up  more 
thoroughly  his  own  department  of  work.  On  the  resignation  of 
President  Gilman  in  1875,  Dr.  Le  Conte  was  induced  again  to 
assume  the  presidency,  which  he  retained  until  June,  1881,  but 
still  performing  the  duties  of  his  professorship.  Since  that  date 
he  has  confined  himself  to  his  chair  of  Physics. 

Through  nearly  the  whole  of  life  the  two  brothers,  John  and 
Joseph  Le  Conte,  have  been  closely  associated,  each  attaining 
eminence,  the  elder  as  a  physicist,  the  younger  as  a  geologist. 
The  elder  preceded  the  younger  by  six  years  at  Franklin  College, 
in  Georgia.  They  went  almost  together  to  the  South  Carolina 
College,  and  likewise  to  the  University  of  California.  This  fact 
has  often  led  to  their  names  becoming  confounded  by  strangers. 

Dr.  Le  Conte  is  a  member  of  the  National  Academy  of  Sciences, 
the  American  Association  for  the  Advancement  of  Science,  the 
American  Philosophical  Society  and  Academy  of  Natural  Sci- 
ences in  Philadelphia,  the  New  York  Academy  of  Sciences,  and 
the  California  Academy  of  Sciences.  To  this  list  might  be  added 
various  other  bodies  which  have  bestowed  upon  him  honorary 
membership. 

A  list  of  some  of  the  more  important  of  Dr.  Le  Conte's  pub- 
lished writings  is  appended.  The  entire  list  is  too  long  for  inser- 
tion, amounting  to  about  a  hundred  papers. 

Of  the  first  dozen,  which  show  the  direction  of  his  tastes  as  a 
physician,  perhaps  the  most  interesting  is  No.  9,  in  which  by  origi- 
nal experiments  he  proved  that  the  alligator  is  able  to  execute  de- 
liberate and  determinate  movements  after  decapitation  and  even 
after  destruction  of  the  spinal  cord. 

In  No.  10  he  shows  that  the  mortality  from  cancer  has  in- 
creased in  modern  times ;  that  it  augments  regularly  with  in- 
creasing age,  and  that  it  is  greater  in  France  than  in  England. 
The  same  subject  is  pursued  still  further  in  No.  28  and  No.  49, 
in  which  he  shows  important  errors  in  the  usual  methods  of  in- 
terpreting vital  statistics,  and  that  the  average  mortality  from 
cancer  is  fully  three  times  as  great  among  females  as  among 
males. 

In  No.  16  he  gives  the  first  rational  explanation  of  a  whole 
class  of  ice  phenomena  as  manifested  both  in  the  ground  and  in 
plants.  In  No.  17  the  investigation  is  continued,  and  from  nu- 
merous experiments  it  is  shown  that  many  plants  may  be  com- 
pletely frozen  without  injury. 

No.  19  is  a  criticism  of  Moseley's  theory  of  the  descent  of 
glaciers,  in  which  it  is  demonstrated  that  the  descent  can  not  be 
produced  by  expansions  and  contractions  of  the  ice  due  to  changes 
of  temperature. 

In  No.  20  it  is  shown  that  Maury's  theory  of  the  winds  is  un- 


SKETCH   OF  PROF.   JOHN  LE   CONTE.  g 

tenable.  This  conclusion  is  now  universally  accepted,  great  as 
was  the  value  of  Maury's  work  in  the  pioneer  days  of  meteorology. 

In  No.  23  it  is  shown  that  solar  light  has  no  sensible  influence 
on  combustion.  This  paper,  as  well  as  Nos.  16  and  17,  was  exten- 
sively reproduced  in  Europe.  The  same  remark  applies  to  Nos. 
34  and  36,  which  have  been  already  discussed. 

In  Nos.  35  and  39  an  account  is  given  of  investigations  regard- 
ing the  depth,  transparency,  and  color-tints  displayed  in  some  re- 
markable bodies  of  water. 

No.  35  contains  the  description  and  discussion  of  some  unique 
experiments  on  the  propagation  of  vibrations  through  water,  the 
source  of  disturbance  being  explosions  of  great  violence.  The  re- 
sults were  wholly  new,  and  attracted  much  attention  in  Europe. 

In  Nos.  37  and  41  the  principles  of  capillarity  are  very  thor- 
oughly discussed,  and  illustrated  by  some  new  experiments. 

Many  others  of  these  papers  might  be  summarized,  but  only 
by  exceeding  the  limits  of  a  brief  biographical  sketch. 

SCIENTIFIC. 

1.  "  Case  of  Carcinoma  of  the  Stomach "  ("  New  York  Medical  Gazette," 
1842). 

2.  "  On  the  Mechanism  of  Vomiting"  ("New  York  Lancet,"  1842). 

3.  "  On  Carcinoma  in  General,  and  Cancer  of  the  Stomach  "  (ibid.,  1842). 

4.  "  On  the  Explanation  of  the  Difference  in  Size  of  the  Male  and  Female 
Urinary  Bladder"  (ibid.,  1842). 

5.  "  An  Essay  on  the  Origin  of  Syphilis  "  ("  New  York  Journal  of  Medical  and 
Collateral  Sciences,"  1844). 

0.  "  Eemarks  on  Cases  of  Inflamed  Knee-Joint"  (ibid.,  1844). 

7.  "  Extraordinary  Effects  of  a  Stroke  of  Lightning. — Singular  Phenomena" 
(ibid.,  1844). 

8.  Observations  on  Geophagy"  (Southern  Medical  and  Surgical  Journal," 
1845). 

9.  "  Experiments  illustrating  the  Seat  of  Volition  in  the  Alligator,  or  Oroco- 
dilus  Lucius  of  Cuvier.  With  Strictures  on  the  Reflex  Theory  "  ("  New  York 
Journal  of  Medical  and  Collateral  Sciences,"  1845  and  1846). 

-—    10.  "  Statistical    Researches  on  Cancer "   ("  Southern  Medical   and   Surgical 
Journal,"  1846). 

11.  "On  the  Quarantine  Regulations  at  Savannah,  Ga."  ("New  York  Journal 
of  Medical  and  Collateral  Sciences,"  1846). 

12.  "Remarks  on  the  Physiology  of  the  Voice  "  ("Southern  Medical  and  Sur- 
gical Journal,"  1846. 

13.  "  Dr.  Bennet  Dowler's  Contributions  to  the  Natural  History  of  the  Alli- 
gator "  (ibid.,  1847). 

14.  "On  Sulphuric  Ether"  (ibid,,  1847). 

15.  "The  Philosophy  of  Medicine:  An  Address  "  (ibid.,  1849). 

16.  "Observations  on  a  Remarkable  Exudation  of  Ice  from  the  Stems  of 
Vegetables,  and  on  a  Singular  Protrusion  of  Icy  Columns  from  Certain  Kinds  of 
Earth  during  Frosty  Weather  "  ("  Proceedings  of  the  American  Association  for  the 
Advancement  of  Science,"  1850  ;  also,  "  Philosophical  Magazine,"  1850). 


lo       SKETCH   OF  PROF.  JOHN  LE   CONTE. 

17.  "  Observations  on  the  Freezing  of  Vegetables,  and  on  the  Causes  which 
enable  some  Plants  to  endure  the  Action  of  Extreme  Cold  "'  ("  American  Journal 
of  Science,"  1852;  also  "Proceedings  of  the  American  Association  for  the  Ad- 
vancement of  Science,"  1851). 

18.  "On  the  Venomous  Serpents  of  Georgia "  ("Southern  Medical  and  Sur- 
gical Journal,"  1853). 

19.  ''  On  the  Descent  of  Glaciers"  ("  American  Journal  of  Science,"  1855). 

20.  "Review  of  Lieutenant  M.  F.  Maury's  Work  on  the  'Physical  Geography 
of  the  Sea  '  "  ("Southern  Quarterly  Review,"  1856). 

21.  "  The  Mechanical  Agencies  of  Heat  "  (ibid.,  1856). 

22.  "  Influence  of  the  Study  of  the  Physical  Sciences  on  the  Imaginative  Fac- 
ulties." An  Inaugural  Address,  delivered  December  1,  1857  (Columbia,  S.  C, 
1858). 

23.  "  Preliminary  Researches  on  the  Alleged  Influence  of  Solar  Light  on  the 
Process  of  Combustion  "  ("  American  Journal  of  Science,"  1857;  also,  "  Proceed- 
ings of  the  American  Association  for  the  Advancement  of  Science,"  1857;  and 
"Philosophical  Magazine,"  1858). 

24.  "  On  the  Influence  of  Musical  Sounds  on  the  Flame  of  a  Jet  of  Coal-Gas  " 
("  American  Journal  of  Science,"  1858  ;   "  Philosophical  Magazine,"  1858). 

25.  "  On  the  Optical  Phenomena  presented  by  the  Silver  Spring  in  Marion 
County,  Florida  (U.  S.),"  ("American  Journal  of  Science,"  1861;  also,  "Pro- 
ceedings of  the  American  Association  for  the  Advancement  of  Science,"  1860). 

26.  "  On  the  Adequacy  of  Laplace's  Explanation  to  account  for  the  Discrep- 
ancy between  the  Computed  and  the  Observed  Velocity  of  Sound  in  Air  and 
Gases  "  ("  Philosophical  Magazine,"  1864). 

27.  "  Limiting  Velocity  of  Meteoric  Stones  reaching  the  Surface  of  the  Earth  " 
("Xature,"  1871). 

28.  "Vital  Statistics:  Illustrated  by  the  Laws  of  Mortality  from  Cancer" 
("  Western  Lancet,"  1872). 

29.  "  Heat  generated  by  Meteoric  Stones  in  traversing  the  Atmosphere  " 
("  Nature,"  1872). 

30.  "The  Nebular  Hypothesis  "  ("  Popular  Science  Monthly,"  1873). 

31.  Articles  on  "Bonanza,"  "  Comstock  Lode,"  and  "Death  Valley,"  in 
"Johnson's  Cycloposdia,"  vol.  iv,  Appendix,  1876. 

32.  "  Mars  and  his  Moons  "  ("  Popular  Science  Monthly,"  1879). 

33.  "Origin  and  Distribution  of  Lakes;  Meteorology  of  the  Pacific  Coast" 
("Mining  and  Scientific  Press  "  and  Supplement,  1880-'81). 

34.  "  Influence  of  Modern  Methods  of  popularizing  Science  "  ("  Berkeleyan," 
1882). 

35.  "  Sound-Shadows  in  Water "  ("  American  Journal  of  Science,"  1882  ; 
also,  "  Philosophical  Magazine,"  1882), 

36.  "Origin  of  Jointed  Structures  in  Undisturbed  Clay  and  Marl  Deposits" 
("American  Journal  of  Science,"  1882). 

37.  "  Apparent  Attractions  and  Repulsions  of  Small  Floating  Bodias  "  ("  Amer- 
ican Journal  of  Science,"  1882  ;  also.  "  Philosophical  Magazine,"  1882). 

38.  "Amount  of  Carbon  Dioxide  in  the  Atmosphere"  ("Philosophical  Maga- 
zine," 1882). 

39.  "  Physical  Studies  of  Lake  Tahoe  "  ("  Overland  Monthlv,"  three  papers, 
188.3-1884). 

40.  "  The  Part  played  by  Accident  in  Discoveries  "  ("Berkeleyan,"  1884). 

41.  "Horizontal  Motions  of  Small  Floating  Bodies,  in  relation  to  the  Validity 


SKETCH   OF  PROF.    JOHN  LE    CONTE.  n 

and  Postulates  of  the  Theory  of  Capillarity  "  ("  American  Journal  of  Science," 
1884;  also,  "Journal  de  Physique,"  1885). 

42.  "  Criticism  of  Bassnett's  Theory  of  the  Sun  "  ("  Overland  Monthly,"  1885). 

43.  "The  Evidence  of  the  Senses  "  ("  J^orth  American  Review,"'  1885). 

44.  "The  Metric  System  "  ("Overland  Monthly,"  1885). 
.   45.  "Thought  Transference"  (ibid.,  1885). 

46.  " Barometer  Exposure  "  ("Science,"  1886). 

47.  "Electrical  Phenomena  on  a  Mountain"  (ibid.,  1887). 

48.  "  Standing  Tiptoe ;  a  Mechanical  Problem  "  (ibid.,  1887). 

49.  "Vital  Statistics,  and  the  True  Coefficient  of  Mortality,  illustrated  by 
Cancer"  ("Tenth  Biennial  Report  of  the  State  Board  of  Health  of  California," 
1888). 

.     50.  "  The  Decadence  of  Truthfulness  "  (1889). 

About  fifty  additional  papers  are  omitted  from  this  list. 


SENSITIVE  FLAMES   AND   SOUND-SHADOWS. 

By  W.  LE  CONTE  STEVENS, 

PROFESSOR   OF  PHYSICS   IN  THB   PACKER   COLLEGIATE   INSTITUTE. 

THE  conception  that  sound  is  due  to  wave-motion  in  an  elastic 
material  medium  was  first  distinctly  expressed  in  the  six- 
teenth century  by  Lord  Bacon.  He  distinguished  between  local 
motion  in  a  medium  and  the  propagation  of  this  motion  through 
it,  referring  to  the  transmission  of  so  and  through  both  air  and 
water  by  way  of  illustration.  For  measuring  the  velocity  of 
sound  in  air  he  proposed  a  plan  which  has  been  repeatedly  applied 
since  his  time,  that  of  firing  a  cannon  and  noting  the  interval  be- 
tween the  flash  and  the  report  as  heard  at  a  measured  distance. 

It  is  impossible  now  to  determine  how  far  these  observations 
may  have  been  original  with  Bacon,  or  to  what  extent  they  may 
have  expressed  the  current  knowledge  of  his  time.  They  were 
clearly  apprehended  by  Galileo,  who  discovered  the  law  of  simple 
harmonic  motion  and  made  the  first  well-authenticated  experi- 
ments on  the  relation  between  vibration  frequency  and  musical 
pitch.  But  it  is  to  Sir  Isaac  Newton  that  we  must  give  the  credit 
of  first  applying  the  wave  theory  rigorously  to  the  phenomena  of 
sound.  Assuming  this  theory,  he  showed  the  possibility  of  calcu- 
lating what  ought  to  be  the  velocity  of  propagation  through  any 
medium  of  known  elasticity.  He  deduced  a  formula  which  has  been 
found  applicable  to  most  media.  In  tlie  case  of  atmospheric  air  it 
failed,  but  because  it  required  a  correction  dependent  on  certain 
laws  of  heat  which  had  not  then  become  known.  The  correction 
was  made  by  Laplace,  and  the  formula,  as  thus  completed,  is  now 
found  to  be  applicable  to  all  known  gases.  This  was  only  one  of 
the  many  important  principles  established  on  a  mathematical  basis 
in  the  "  Principia,"  and  published  in  1687. 

Even  before  this  date,  the  conception  that  light,  as  well  as 
sound,  might  be  due  to  wave-motion  seems  to  have  been  grasped 
by  a  few  thinkers.  In  16G5  a  book  on  "  Light  and  Color  "  was  pub- 
lished at  Bologna,  two  years  after  the  death  of  its  author,  Fran- 
cesca  Maria  Grimaldi,  a  Jesuit  priest  and  astronomer.  In  this  he 
recounts  some  interesting  experiments,  which  did  not,  it  is  true, 
lead  him  to  the  wave  theory  of  light,  but  served  as  the  basis  on 
which  this  theory  was  subsequently  established.  Similar  experi- 
ments were  made  soon  afterward  by  Kobert  Hooke,  the  ever- 
jealous  rival  of  Newton,  and  by  Christian  Huygens,  their  distin- 
guished Dutch  contemporary.  Huygens  demonstrated  that,  if  an 
impulse  be  given  to  any  single  particle  in  a  uniformly  elastic  ma- 
terial medium,  it  must  be    propagated  thence  as  wave-motion 


SENSITIVE  FLAMES  AND   SOUND-SHADOWS.        13 

equally  in  all  directions ;  and  that  the  propagation  of  a  wave  front 
in  any  given  direction  is  the  result  of  a  multitude  of  interferences 
among  the  elementary  waves  started  from  the  particles  which  are 
successively  disturbed.  Accepting  this  principle,  the  laws  of 
reflection  and  refraction,  whether  of  light  or  sound,  follow  imme- 
diately ;  and  they  were  worked  out  with  great  skill  by  Huygens. 
Another  consequence  is,  that  if  an  obstacle  be  interposed  in  the 
path  of  a  wave,  its  edges  must  serve  as  new  centers  around  which 
secondary  waves  will  be  propagated,  while  the  main  wave  con- 
tinues to  advance.     This  is  familiar  in  the  case  of  water-waves. 

If,  therefore,  light  be  due  to  wave-motion,  no  perfect  geo- 
metric shadow  is  possible,  for  the  shadow  must  suffer  encroach- 
ment from  these  secondary  waves  thus  diffracted.  Such  phenom- 
ena were  actually  observed  in  the  case  of  light  by  Grimaldi, 
Hooke,  and  Huygens,  but  no  satisfactory  explanation  was  then 
given.  It  is  surprising  that  Huygens  did  not  think  of  applying 
the  theory  which  had  been  so  satisfactory  in  its  application  to 
other  optical  phenomena.  He  had  not  attempted  to  measure  the 
length  of  waves  of  light,  and  had  no  conception  of  their  exceeding 
minuteness.  If  any  diffraction  phenomena  were  to  be  observed, 
the  encroachment  for  which  he  naturally  looked  was  far  greater 
than  what  had  been  noticed  as  inexplicable  and  almost  impercept- 
ibly narrow  fringes.  The  absence  of  the  diffraction  phenomena 
such  as  he  may  have  expected  did  not  cause  him  to  abandon  his 
wave  theory,  though  he  could  not  but  perceive  that  it  constituted 
a  stumbling-block.  To  the  mind  of  Newton  this  obstacle  was  in- 
superable ;  it  determined  his  rejection  of  Huygens's  theory. 

If  Newton  was  not  the  inventor  of  the  emission  theory  of  light, 
he  was  certainly  its  most  ardent  advocate.  It  came  into  promi- 
nence along  with  the  wave  theory,  or  indeed  a  little  after  this ; 
and  by  means  of  it  very  satisfactory  explanations  could  be  given 
of  most  optical  phenomena.  Newton's  reasoning,  and  the  author- 
ity of  his  great  name,  caused  its  acceptance  by  all  contemporary 
physicists,  except  Hooke,  Huygens,  and  Euler,  and  by  all  his  suc- 
cessors for  a  century.  Whichever  of  the  two  theories  is  accepted, 
assumptions  are  involved  which  are  open  to  attack  and  incapable 
of  being  substantiated  on  any  antecedent  grounds.  Its  value  has 
to  be  measured  alone  by  its  consistency  with  observed  facts.  It 
was  not  until  about  the  beginning  of  the  present  century  that  Dr. 
Thomas  Young  revived  the  long-discarded  wave  theory,  explained 
the  diffraction  of  light  by  its  aid,  and  showed  the  incompetency 
of  the  emission  theory.  His  views  were  at  first  generally  rejected, 
but  in  time  they  attracted  the  attention  of  Arago  and  Fresnel.  The 
latter  especially  entered  into  the  investigation  with  enthusiasm^ 
and  completed  the  establishment  of  the  wave  theory  upon  founda- 
tions that  have  never  since  been  successfully  assailed.    The  elastic 


14        SENSITIVE  FLAMES   AND    SOUND-SHADOWS. 

medmm  required  for  the  propagation  of  light-waves,  whether 
through  interplanetary  space  or  terrestrial  bodies,  is  the  universal 
ether,  of  whose  existence  we  have  no  evidence  except  that,  by  as- 
suming it  and  applying  mathematics,  the  results  of  computation 
are  exactly  corroborated  by  observation  and  experiment.  The 
elastic  medium  required  for  sound-waves  may  be  solid,  liquid,  or 
gaseous.     In  any  case  it  must  be  material. 

Assuming,  then,  an  obstacle  in  the  path  of  a  wave  of  sound 
or  light,  a  shadow  should  be  produced ;  but  since  the  edges  are 
sources  of  secondary  waves,  according  to  Huygens's  principle, 
these  should  encroach  upon  the  shadow.  The  degree  of  encroach- 
ment can  be  expressed  in  a  mathematical  formula,  and  is  thus 
shown  to  be  proportional  to  the  wave-length.  The  average  length 
of  a  wave  of  green  light  is  now  known  to  be  about  -g-rJrr  of  an 
inch.  The  encroachment  on  the  geometric  shadow  is  hence  so 
small  that  refined  methods  are  needed  to  make  it  perceptible.  In 
the  case  of  audible  sound,  on  the  contrary,  when  propagated 
through  air,  the  wave-length  is  ordinarily  so  great  that  the  en- 
croachment almost  wholly  masks  the  presence  of  any  shadow 
whatever.  For  the  pitch  C,  132  vibrations  per  second,  such  as  is 
often  used  by  men  in  conversation,  the  wave-length  is  readily  cal- 
culated, if  we  know  the  velocity  of  sound  in  air.  Taking  this  as 
1,120  feet  per  second,  there  will  be  132  waves  strung  out  over  this 
distance  in  each  second.  The  length  of  each  is  hence  eight  feet 
and  six  inches,  or  more  than  five  million  times  as  great  as  that 
of  the  average  wave  of  light.  For  such  waves  it  is  hopeless  to 
attempt  producing  any  well-defined  shadows. 

One  of  the  most  familiar  facts  in  physics  is  that  the  pitch  of  a 
note  becomes  higher,  and  hence  its  wave  shorter,  in  proportion  to 
the  increase  of  vibration  frequency.  If  well-defined  sound-shad- 
ows are  possible,  we  must  resort  to  sounds  of  very  short  wave- 
length. If  the  sound  is  continuous  instead  of  explosive,  this  short- 
ness implies  very  high  pitch.  There  are  mechanical  difficulties 
to  contend  with  which  make  it  hard  to  give  much  intensity  to 
very  acute  sounds.  The  range  of  audition,  moreover,  is  limited. 
For  persons  of  good  ear  the  range  may  be  roughly  stated  as  from 
25  to  25,000  vibrations  per  second  for  sounds  of  small  intensity ; 
indeed,  many  fail  to  perceive  any  pitch  exceeding  15,000.  To 
exhibit  sound-shadows,  therefore,  it  becomes  necessary  either  to 
employ  a  source  that  sends  forth  sounds  of  such  high  pitch  as  to 
be  inaudible  to  most  of  those  who  are  expected  to  perceive  the 
shadow,  or  to  resort  to  a  momentary  sound  of  great  intensity  and 
short  wave-length. 

Every  one  has  noticed  the  decrease  in  intensity  of  the  sound  of 
a  distant  railway-train  as  it  passes  into  a  cutting.  The  observer 
is  in  a  shadow  which  is  incomplete  but  nevertheless  noticeable. 


SENSITIVE  FLAMES  AND   SOUND-SHADOWS.         15 

The  secondary  waves,  which,  are  started  at  the  upper  edges  of  the 
cutting,  reach  the  ear  and  give  still  a  good  idea  of  the  character 
of  the  noise  and  position  of  the  train.  The  range  of  the  ear  greatly 
exceeds  that  of  the  eye,  not  only  in  relation  to  the  variety  of  wave- 
lengths by  which  it  may  be  impressed,  but  yet  more  as  to  varia- 
tions of  intensity.  Just  as  sunshine  and  shadow  during  the  day 
indicate  merely  variations  in  illumination  without  the  complete 
extinction  of  light,  so  noise  and  sound-shadow  are  merely  relative 
terms,  the  latter  not  necessarily  implying  the  complete  extinction 
of  sound ;  for  in  air  diffraction  usually  plays  so  important  a  part 
as  to  forbid  complete  extinction,  and  to  prevent  all  sharpness  of 
definition  at  the  edges  of  the  shadow. 

When  the  medium  is  water  instead  of  air,  some  new  phenomena 
are  noticeable.  In  1836  Daniel  Colladon's  classic  experiments  on 
the  velocity  of  sound  in  water  were  performed  on  the  Lake  of  Ge- 
neva. The  source  of  sound  was  a  large  bell,  from  which  vibra- 
tions were  conducted  through  the  water  several  miles  away  to  an 
elastic  membrane  stretched  across  the  expanded  opening  of  a  par- 
tially submerged  hearing-trumpet.  They  were  thus  given  to  the 
air  within  the  trumpet  and  conveyed  to  an  ear  applied  at  its  smaller 
end  above  the  water.  A  bell  when  struck  sends  forth  a  vari- 
ety of  tones,  and  it  is  often  hard  to  determine  which  of  these  is 
most  prominent.  Usually  that  of  deepest  pitch  is  the  slowest  to 
die  away  in  air,  and  often  it  penetrates  to  the  greatest  distance. 
Colladon  made  the  remarkable  observation  that  in  water  the  low- 
er tones  are  conducted  oif  to  but  a  short  distance  before  their 
energy  ceases  to  produce  the  sensation  of  sound ;  while  the  initial 
stroke  is  propagated  much  further,  and  is  then  perceived  as  a 
short,  sharp,  almost  clicking  sound,  without  definite  musical  char- 
acter. Placing  the  hearing-trumpet  behind  a  wall  which  project- 
ed out  into  the  water,  the  decrease  of  intensity  was  much  greater 
than  under  similar  conditions  in  air,  and  the  demarkation  of  the 
region  of  shadow  was  decidedly  more  noticeable. 

Still  more  interesting  than  the  experiments  of  Colladon  were 
those  made  in  the  Bay  of  San  Francisco  in  1874  by  Prof.  John 
Le  Conte  and  his  son,  Mr.  Julian  Le  Conte.  The  source  of  sound 
was  not  such  as  would  give  a  definite  pitch,  like  a  bell,  but  the 
quick,  violent,  single  impulse  due  to  the  explosion  of  dynamite 
employed  in  the  blasting  of  rocks  which  obstructed  the  channels. 
The  intensity  of  the  shock  thus  propagated  was  such  as  to  be  felt 
as  a  blow  on  the  feet  of  a  person  seated  in  a  boat  three  hundred 
feet  or  more  from  the  detonating  cartridge,  and  to  kill  hundreds 
of  fish.  Several  vertical  posts  or  piles,  each  about  a  foot  in  diame- 
ter, projected  from  the  ground  out  of  the  water  in  the  neighbor- 
hood. A  stout  glass  bottle  was  suspended  in  the  water  about  a 
foot  in  the  rear  of  one  of  these  piles  (Fig.  1),  within  the  geomet- 


i6 


SENSITIVE  FLAMES   AND    SOUXD-SHADOWS. 


ric  shadow  determined  by  lines  supposed  to  be  drawn  from  tbe 
cartridge  forty  feet  horizontally  away.  The  bottle  was  perfectly 
protected  from  the  shock  of  the  explosion.  It  was  tlien  put  in 
front  of  the  pile.  The  first  shock  shivered  it  into  hundreds  of 
fragments.     Other  bottles,  some  filled  with  air  and  some  with 


Fig.  I. 

water,  were  similarly  exposed  in  various  directions  around  the 
pile,  and  with  the  same  result — destruction,  except  when  within 
the  protecting  shadow.  The  experiments  were  varied  by  immers- 
ing stout  glass  tubes  (Fig.  1),  incased  in  thick  paper,  horizontally 
across  the  direction  of  the  sound-rays  in  water,  between  two  piles 
which  were  aligned  with  the  dynamite  cartridge.    These  piles  were 

twelve  feet  apart,  the 
nearer  one  being  forty 
feet  from  the  cartridge. 
Its  shadow,  therefore, 
just  covered  the  second 
pile,  and  included  the  in- 
termediate water,  with 
the  middle  part  of  each 
tube.  After  an  explosion 
these  protected  parts 
were  found  to  be  un- 
broken, while  the  ends 
which  projected  on  the  two  sides  beyond  the  shadow  were  com- 
pletely shattered  (Fig.  2).  The  boundary  between  the  regions  of 
shadow  and  noise  was  sharply  defined  on  the  tubes,  even  at  a  dis- 
tance of  twelve  feet  behind  the  protecting  pile. 

To  account  for  the  shortness  of.  the  sound-waves  which  were 


Fig.  2. 


SENSITIVE  FLAMES  AND   SOUND-SHADOWS.        17 

capable  of  producing  such  sharp  shadows,  Dr.  Le  Conte  advances 
what  seems  to  be  the  only  tenable  theory,  and  one  which  equally 
explains  the  observations  of  Colladon  on  the  clicking  sound  of  a 
distant  bell  as  heard  in  water.  In  the  absence  of  any  recogniz- 
able pitch — for  pitch  implies  a  series  of  impulses  recurring  in 
regular  order — there  is  no  means  of  determining  wave-length  in 
these  cases.  But  whatever  this  may  be,  the  wave-length  is  equal 
to  the  product  of  the  time  consumed  in  generating  the  wave  and 
the  velocity  of  propagation.  Thus,  assume  the  initial  pitch  of  a 
bell  to  be  220  vibrations  per  second.  We  may  compute  the  wave- 
length either  by  considering  that  220  waves  are  strung  out  over  a 
distance  of  1,120  feet,  making  each  a  trifle  more  than  five  feet 
long,  or  we  may  say  that  the  time  consumed  in  generating  each 
wave  is  ^|-g-  of  a  second,  and  that  this  impulse  is  propagated  at 
the  rate  of  1,120  feet  per  second,  which  would  be  a  little  over  five 
feet  in  ^^r  of  a  second.  The  blow  of  the  hammer  on  Colladon's 
bell  was  almost  instantaneous,  and  the  intensity  of  the  first  shock 
thus  given  to  the  water  was  far  greater  than  that  of  any  subse- 
quent shock  due  to  the  succession  of  vibrations  set  up  in  the  elas- 
tic bell-metal.  The  distance  through  which  this  intense  sound 
would  be  propagated  might  be  expected  greatly  to  exceed  that 
traversed  by  the  subsequent  weaker  vibrations.  The  generating 
blow  was  so  brief  that  the  wave-length  could  only  be  short ;  and 
hence  comparatively  well-defined  sound-shadows  were  produced 
at  a  distance.  In  the  case  of  the  dynamite  explosions  under  water 
this  reasoning  holds  with  yet  greater  force.  If  the  duration  of 
the  generating  impulse  be  only  a  millionth  of  a  second,  and  the 
velocity  of  propagation  in  water  be  4,700  feet  per  second,  the 
resulting  wave-length  would  be  only  about  -^  of  an  inch.  The 
quickness  of  action  manifested  in  the  explosion  of  dynamite  ex- 
ceeds that  of  any  other  known  agent  that  has  ever  been  similarly 
employed.  The  duration  of  the  generating  impulse  may  be  con- 
sidered indefinitely  small,  certainly  immeasurably  small.  The 
sharpness  of  the  sound-shadows  it  produces  in  water  indicates  a 
wave-length  that  can  not  exceed  a  small  fraction  of  an  inch. 

The  production  of  sharp  sound-shadows  in  air  is  of  more 
recent  date  than  the  experiments  in  water.  In  1880  a  dynamite- 
factory  near  San  Francisco  was  destroyed  by  the  explosion  of  its 
contents.  On  a  large  building  three  miles  away  many  panes  of 
window  glass  on  the  side  toward  the  explosion  were  broken,  and 
two  shocks  were  felt,  one  conducted  by  the  air  and  the  other  by 
the  ground.  In  the  acoustic  shadow  cast  by  this  building,  nearly 
nine  hundred  feet  away  on  the  side  remote  from  the  explosion,  no 
aerial  shock  was  experienced,  though  that  from  the  ground  was 
distinctly  felt.  The  shortness  of  the  air-wave  due  to  exploding 
dynamite  sufficiently  accounts  for  the  sharpness  of  the  shadow. 


i8        SENSITIVE  FLAMES   AND    SOUND-SHADOWS. 


But  there  is  now  no  longer  any  necessity  to  resort  to  such 
dangerous  sources  of  sound  as  dynamite.  Whistles  may  be  made 
which  yield  tones  exceeding  twenty  thousand  vibrations  per  sec- 
ond. The  wave-length  corresponding  to  such  a  pitch  is  less  than 
an  inch.  The  advantage  presented  is  that  the  sound  is  continu- 
ous, and  it  may  be  made  as  constant  as  we  please  by  supplying 
the  whistle  from  a  cylinder  full  of  compressed  air,  regulating  the 
pressure  by  means  of  an  appropriate  gauge.  The  disadvantage  is 
that  the  intensity  is  but  slight,  and  the  pitch  is  too  high  to  be 
perceived  as  sound  by  most  persons  unless  the  ear  is  closely  ap- 
plied. An  artificial  indicator  must  hence  be  used,  whose  motion 
under  the  disturbances  due  to  sound  can  be  seen  at  a  distance. 

In  1857  Prof.  John  Le  Conte  discovered  that  an  ordinary  naked 
gas-flame,  from  a  fish-tail  or  bat-wing  burner,  becomes  an  indicator 
of  sound  by  vibrating  in  unison  with  an  external  source,  provided 
the  pressure  be  such  that  the  flame  is  just  ready  to  flare.  This 
can  be  easily  shown  by  blowing  a  shrill  whistle  or  bowing  a 
tuning-fork  of  high  pitch  in  the  immediate  neighborhood  of  the 
flame,  which  at  once  becomes  forked  (Fig.  3)  into  several  long, 

vibrating  tongues.  The 
effect  soon  ceases  if  the 
pressure  be  gradually  di- 
minished. This  result  is 
due  to  the  disturbance 
produced  by  sound  - 
waves  on  the  outflowing 
jet  of  gas  at  the  nozzle. 
The  high  temperature  of 
flame  is  therefore  not 
necessary  for  the  pro- 
duction of  such  co-vibra- 
tion, but  serves  to  make 
it  more  easily  manifest. 
Nine  years  elapsed  after  Dr.  Le  Conte's  discovery  before  the 
subject  was  taken  up  again  and  independently  by  Mr.  W.  F. 
Barrett,  in  London,  who  used  small  cylindrical  jets,  which  were 
found  to  flare  under  similar  conditions,  and  could  be  ren- 
dered far  more  sensitive.  A  "  pin-hole  lava-tip  "  may  be  fitted 
into  the  end  of  a  metal  tube  and  connected  by  means  of  India- 
rubber  tubing  to  a  cylinder  of  compressed  illuminating  gas.  In 
connection  with  this,  also,  there  should  be  a  water  manometer 
gauge  for  regulating  the  pressure  of  the  outflowing  gas.  If  the 
pin-hole  is  very  smoothly  cylindrical,  the  flame  mounts  up  to  the 
height  of  nearly  eighteen  inches  (Fig.  4,  x),  with  an  apparent 
thickness  scarcely  more  than  that  of  the  little  finger,  and  burn- 
ing quietly.     When  the  pressure  approaches  ten  inches,  as  indi- 


Fis.  3.— Sensitive  Batwing  Flames. 


SENSITIVE  FLAMES  AND   SOUND-SHADOWS.       19 


cated  by  the  water-gauge,  the  flame  flares,  becoming  much  shorter 
and  broader,  like  a  little  Indian  club  (Fig.  4,  y),  and  producing  a 
low  roaring  sound,  due  to  the  escape  of  unburned  gas.  Let  the 
pressure  now  be  diminished  until  this  flaring  barely  ceases.  The 
flame  is  now  in  its  most  sensitive  condition.  Sounds 
of  low  or  even  medium  pitch  have  no  efliect  upon  it ; 
but  on  blowing  a  shrill  whistle,  or  rattling  a  bunch 
of  keys  anywhere  within  thirty  or  forty  feet,  it  flares. 
Perhaps  the  most  beautiful  illustration  of  its  sensi- 
tiveness is  given  by  placing  an  open  watch  near  the 
nozzle  but  not  touching  it ;  every  tick  causes  a  mo- 
mentary sinking  and  spreading  of  the  flame,  so  that 
the  effect  may  be  seen  across  an  audience-room.  If 
the  audience  applauds  with  clapping  of  hands,  the 
flame  shrinks  in  acknowledgment. 

A  very  sensitive  flame,  but  not  so  convenient  as 
that  of  Prof.  Barrett,  and  not  visible  at  so  great  a 
distance,  may  be  obtained  with  no  pressure  greater 
than  that  of  the  street  mains,  by  causing  the  gas  to 
issue  from  a  small  tube,  over  the  orifice  of  which,  at 
a  height  of  an  inch  or  two,  is  placed  a  piece  of  wire 
gauze  (Fig.  5).  The  mixture  of  coal-gas  and  air  is 
ignited  above  the  gauze,  and  a  glass  tube  may  be  used 
to  protect  the  flame  from  currents  of  air,  though  this 
is  not  usually  necessary.  Very  little  adjustment  is 
needed  to  find  the  distance  between  nozzle  and  gauze 
at  which  the  flame  is  most  sensitive.  This  arrange- 
ment was  devised  independently  by  Prof.  Govi,  of 
Turin,  and  Mr.  Barry,  of  Ireland.  The  flame  is  de- 
ficient in  brightness,  and  is  only  a  few  inches  high  at 
its  best,  but  has  the  advantage  of  not  requiring  any 
appliances  that  may  not  be  easily  supplied  in  any 
town.  If  Barrett's  flame  is  available,  however,  it  is 
decidedly  preferable  to  anything  else. 

With  such  a  flame  as  Barrett's  it  becomes  possible 
to  explore  the  air  and  detect  regions  of  relative  noise  and  silence* 
just  as  a  delicate  thermometer  enables  us  to  determine  variations  of 
temperature  in  different  layers  of  air  or  water.  If  the  pitch  be  too 
high  for  the  ear  to  estimate  or  even  detect  it,  the  sensitive  flame 
is  more  delicate  than  the  ear.  Armed  with  a  whistle  yielding  a 
pitch  of  twelve  or  fifteen  thousand  vibrations  per  second,  and  with 
a  good  flame,  many  beautiful  analogies  between  sound  and  light 
may  be  exhibited  with  entire  satisfaction  to  an  audience  of  deaf- 
mutes,  if  the  lecturer's  fingers  are  fairly  nimble,  since  there  is  no 
necessity  for  the  sounds  to  be  heard.  Most  of  the  experiments 
about  to  be  described  were  devised  by  Lord  Rayleigh,  the  suc- 


FiQ.  4. 


20 


SE.YSITIVE  FLAMES   AND    SOUND-SHADOWS. 


cesser  of  Prof.  Tyndall  in  tlie  chair  of  Natural  Philosopliy  at  the 
Royal  Institution  in  London. 

Let  the  whistle  be  supplied  with  a  continuous  blast  of  air,  or 
any  compressed  gag,  at  steady  pressure.  Four  or  five  feet  away 
from  it  is  placed  the  nozzle  of  the  burner  from  which  the  flame 
issues.  Its  sensitiveness  may  be  regulated  at  will  by  means  of  the 
stop-cock  and  the  water  manometer  gauge.  Turning  on  the  blast 
through  the  whistle,  the  flame  flares.  Let  the  open  hand  be  held 
up  between  the  two ;  the  flaring  ceases.  The  nozzle  of  the  burner 
is  in  the  acoustic  shadow  cast  by  the  hand.  If  this  result  is  not 
successfully  attained  at  the  first  trial,  the  sensitiveness  of  the 
flame  may  be  slightly  modified  to  suit  the  conditions.  The  case 
is  entirely  analogous  to  that  of  the  glass  bottles  in  the  experi- 
ments in  San  Francisco  Bay. 

By  using  a  small  mirror  to  reflect  the  sound-waves,  their 
lengths  may  easily  be  measured  in  mid-air.  Let  the  mirror  be 
put  a  few  inches  behind  the  flame  and  moved  slowly  toward  this 
or  away  from  it.  At  certain  distances  the  flame  is  observed  to 
flare  violently,  and  at  certain  other  points  it  becomes  quiet, 
though  the  sound  has  not  been  varied.  Reflected  waves  are 
meeting  advancing  waves.  Where  they  meet  in  like  phases,  their 
effect  on  the  flame  is  intensified.  But  if  the  position  of  the  mirror 
is  so  adjusted  that  the  flame  is  at  a  point  where  the  opposing 

waves  meet  in  unlike  phase,  these 
neutralize  each  other  and  the  flame 
ceases  to  be  agitated.  The  case  is  like 
that  of  producing  loops  and  nodes  on 
a  string  attached  at  one  end  to  a  vi- 
brating body  and  fixed  at  the  other 
end.  A  series  of  sinusoidal  curves 
travel  over  its  length,  and  are  re- 
flected from  the  fixed  end,  producing 
the  so-called  stationary  waves  (Fig. 
0).  A  returning  sinusoid  is  super- 
imposed on  an  advancing  sinusoid, 
producing  two  loops,  with  an  inter- 
mediate nodal  point  of  rest  and  a 
node  at  the  end.  The  whole  sinusoid 
represents  a  wave-length,  and  the 
distance  from  node  to  node  a  half 
wave  -  length.  The  distance  through 
which  the  mirror  is  moved  from  one  point  of  flame  quiescence 
to  the  next  is  a  half  wave-length  for  the  pitch  yielded  by  the 
whistle.  In  some  experiments  thus  conducted  by  the  writer, 
this  distance  was  found  to  be  a  trifle  over  half  an  inch.  The 
whole  wave-length  was  1*05   inch.     Assuming   the  velocity  of 


Fig.  5. 


SENSITIVE  FLAMES  AND    SOUND-SHADOWS. 


21 


Pig.  6.— o,  advancing  sinusoid;  6,  returnins  Binuaoid; 
c,  advancing  and  returning  ^inuBoids,  forming  two 
loops  and  a  node  ;  c  «  is  a  whole  wave-length ;  c  d,& 
half  wave-lengih. 


sound  to  be  1,120  feet,  reducing  tliis  to  inches,  and  dividing  by 
1*05  inch,  the  pitch  of  the  whistle  was  thus  found  to  be  in  the 
neighborhood  of  thirteen  thousand  complete  vibrations  per  sec- 
ond. In  no  other  way  could  this  pitch  be  determined,  for  the 
most  accomplished  musician  loses  his  power  of  discriminating 
pitch  when  either  the  upper  or  the  lower  limit  of  audition  is  ap- 
proached. The  pitch  of  the 
highest  tone  employed  in 
music  does  not  exceed  five 
thousand  vibrations  per 
second. 

In  performing  this  ex- 
periment Lord  Rayleigh 
discovered  an  interesting- 
peculiarity  of  the  human 
ear  in  contrast  with  the 
sensitive  flame.  By  using 
a  tube,  whose  opening  was 
placed  alternately  in  the 
aerial  loops  and  nodes,  and 
conveying  the  sound  thus  to  the  ear  at  the  same  time  that  the  flame 
was  alternately  agitated  and  quiescent,  he  found  that  the  ear  was 
most  affected  where  the  flame  was  least  affected,  and  vice  versa. 

The  flame,  moreover,  is  unequally  sensitive  in  two  directions 
at  right  angles  with  each  other.  In  drilling  the  small  cylindrical 
hole  of  the  burner  no  amount  of  care  is  sufficient  to  prevent 
minute  irregularities.  The  current  of  issuing  gas  is  not  abso- 
lutely cylindrical.  It  is  disturbed  slightly  by  interior  currents 
from  side  to  side,  and  these  affect  the  sensitiveness  of  the  jet  to 
external  disturbances.  To  test  this,  let  the  nozzle  be  rotated  on  its 
own  axis  while  the  whistle  is  sounding,  until  the  maximum  effect 
is  noticed  ;  and  let  the  sensitiveness  of  the  flame  be  slightly  re- 
duced without  causing  it  to  cease  to  flare.  On  rotating  the 
nozzle  now  through  a  right  angle  the  flame  is  found  to  become 
quiet.  Let  a  mirror  be  put  on  one  side  of  the  flame,  a  short  dis- 
tance off,  so  as  to  face  the  sensitive  side.  Adjusting  it  until  it  is 
equally  inclined  to  the  directions  of  flame  and  whistle,  the  flaring 
is  started  anew.  This  ceases  when  the  mirror  is  rotated  toward 
either  side  through  a  very  small  angle.  Indeed,  no  more  beautiful 
and  exact  illustration  could  be  devised  for  showing  the  law  of 
reflection  of  sound-waves.  The  sound-ray,  taking  a  longer  and 
broken  path,  disturbs  the  flame  on  its  sensitive  side,  while  the 
direct  rays  are  at  the  same  time  beating  in  vain  against  what  by 
analogy  we  may  call  its  deaf  side. 

Probably  the  most  interesting  acoustic  phenomena  to  be  in- 
vestigated by  the  aid  of  the  sensitive  flame  are  those  of  diffrac- 


22        SENSITIVE  FLAMES  AND   SOUND-SHADOWS. 

tion,  or  the  measurable  encroachment  upon  sound-sliadows.  In  the 
accompanying  diagram  (Fig.  7)  suppose  the  arrows  to  represent 
the  direction  of  a  group  of  parallel  rays  of  either  sound  or  light, 
the  wave  fronts  being  indicated  by  lines  across  the  direction  of 
the  arrows.  Waves  in  one  phase  are  indicated  by  the  continuous 
lines,  and  those  in  opposite  phase  by  the  dotted  lines.  At  each 
edge  of  the  obstacle  are  the  centers  of  the  secondary  waves,  whose 
fronts  are  represented  by  parts  of  circles.  Behind  the  obstacle 
and  on  each  side  are  points  of  interference  represented  by  crosses 


Fio.  7. — Exterior  and  Interior  DirrRACTioN. 


and  zeros.  Behind  it  the  secondary  waves  from  opposite  edges 
meet  each  other.  At  the  sides,  secondary  waves  interfere  with 
the  advancing  main  wave.  Where  like  phases  meet,  the  crosses 
represent  points  of  increased  disturbance.  Where  opposite  phases 
meet,  the  zeros  represent  points  of  quiescence.  If  the  waves  are 
those  of  light,  the  crosses  are  points  of  increased  brightness  ;  the 
zeros,  of  comparative  darkness.  If  the  waves  are  those  of  sound, 
the  crosses  are  points  of  noise ;  the  zeros,  of  silence.  Behind  the 
obstacle  there  is  a  middle  line  of  crosses ;  on  each  side  of  this  a 
line  of  zeros;  and  outside  of  these  are  lines  of  crosses  again. 
These  lines  are  parts  of  hyperbolas,,  whose  foci  are  the  centers 
from  which  the  secondary  waves  are  started.  This  is  readily  seen 
by  reference  to  the  next  illustration  (Fig.  8).  A  necessary  conse- 
quence is,  that  if  light  radiating  from  a  point  or  a  small  aperture 
be  interrupted  by  interposing  a  small  disk  in  its  path,  there  should 
be  a  line  along  the  middle  of  the  shadow  behind  it,  at  certain 
points  of  which  brightness  appears  if  a  translucent  screen  is  placed 
across  the  shadow.  This  fact  was  noticed  by  a  Frenchman,  De- 
lisle,  before  the  birth  of  either  Newton  or  Huygens,  but  was  of 
course  not  understood  and  was  soon  forgotten.  Dr.  Young  seems 
not  to  have  thought  of  it,  or  certainly  never  put  this  consequence 


SENSITIVE  FLAMES  AND   SOUND-SHADOWS.        23 


of  theory  to  any  test.  The  first  physicist  to  recognize  the  value 
of  Young's  optical  papers  was  Arago,  who  at  once  adopted  the 
wave  theory  and  started  his  friend  Fresnel  on  a  series  of  optical 
researches  that  are  now  classic.  In  1819  Fresnel  gained  a  prize 
from  the  French  Academy  for  his  work  on  diffraction  of  light. 
Before  the  report  was  made  to  the  Academy  it  was  examined  by 
the  mathematician  Poisson,  who  criticised  it  by  showing  that,  if 
the  wave  theory  were  ac- 
cepted, the  shadow  of  a 
small  disk  should  have  a 
bright  spot  in  the  middle, 
due  to  diffraction,  the  illu- 
mination of  which  should 
be  the  same  as  if  no  disk 
had  been  interposed.  Ara- 
go at  once  tried  the  ex- 
periment ;  and  what  Pois- 
son had  urged  to  prove  the 
impossibility  of  Fresnel's 
views  was  found  to  be  a 
startling  proof  of  their 
correctness.  The  experi- 
ment is  easily  tested,  re- 
quiring no  more  expensive 
apparatus  than  a  mirror 
outside  of  an  opening  in 
a  window,  a  small  bullet 
suspended  by  a  thin  wire, 
and  a  piece  of  rough- 
ened glass  to  receive  the 

shadow.  A  pin-hole  through  a  sheet  of  tin  foil  covering  the  win- 
dow opening  yields  the  required  light  from  the  mirror.  The 
acoustic  analogue  of  this  celebrated  experiment  was  first  accom- 
plished a  few  years  ago  by  Lord  Rayleigh;  it  has  been  lately 
often  repeated  by  the  writer  and  perhaps  others.  A  disk  of  card- 
board about  a  foot  in  diameter  is  put  between  a  whistle  and  sensi- 
tive flame,  with  careful  adjustment  of  distance  and  sensitiveness. 
In  certain  positions  the  flame  is  protected  within  the  shadow  of 
the  disk ;  but,  by  moving  the  latter  to  and  fro,  one  position  is 
found  where  it  causes  the  flame  to  be  violently  agitated  by  the 
meeting  of  waves  diffracted  at  the  edge  of  the  circle.  The  diffract- 
ive  effect  is  the  same  as  if  the  impervious  disk  were  a  lens  con- 
verging the  sound-waves  to  a  focus. 

The  effect  just  described  may  be  much  intensified  by  construct- 
ing an  acoustic  diffraction  grating  and  using  it  in  place  of  the 
simple  disk.     The  explanation  of  the  principle  on  which  such  a 


s 

Fio.  8 


4'       3"       >'     *"       9        1         J^   f  S      ^ 

Hyperbolas  prodfced  by  Interfebencb  of 

Waves. 


24        SENSITIVE  FLAMES  AND   SOUND-SHADOWS. 

grating  is  made  is  beyond  the  scope  of  the  present  paper.*  As- 
suming its  use,  the  sensitive  flame  enables  us  to  detect  a  focal 
area  of  noise,  at  which  the  flame  is  violently  agitated,  and  around 
this  are  alternate  rings  of  silence  and  fainter  noise  diminishing 
in  strength  with  increase  of  distance  from  the  central  focus. 

By  admitting  light  through  two  small  openings  close  together, 
the  waves  coming  from  a  distant  bright  point  and  hence  reaching 
the  two  openings  in  the  same  phase,  hyperbolic  lines  of  interfer- 
ence like  those  shown  in  Fig.  8  were  traced  in  space  by  Fresnel. 
The  writer  has  recently  done  the  same  with  sound-waves,  using 
the  sensitive  flame  as  an  explorer.  Bands  of  alternate  noise 
and  silence  have  in  like  manner  been  traced  by  him  in  air,  pro- 
duced by  interference  between  the  waves  x>i'oceeding  directly 
from  the  whistle  and  those  reflected  from  a  smooth  surface  placed 
horizontally  on  the  table. 

The  wave  theory  of  sound  has  long  been  impregnable;  but 
these  beautiful  analogies  between  light  and  sound,  though  pro- 
vided for  by  theory,  have  been  experimentally  demonstrated  only 
recently.  Such  new  and  unexpected  confirmations,  new  points  of 
contact,  are  always  welcome,  even  though  they  be  not  needed  for 
the  establishment  of  a  theory.  They  are  the  results  of  prevision 
based  on  the  assumption  that  an  elastic  material  medium  is 
needed  for  the  propagation  of  sound,  and  are  wholly  inexplicable 
on  any  theory  of  emanation  analogous  to  Newton's  emission  the- 
ory of  light. 

*  For  this  explanation  the  reader  is  referred  to  an  article  on  "  Diffraction  of  Sound,"  in 
the  "Journal  of  the  Franklin  Institute,"  for  June,  1889. 


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